Nonperturbative and spin effects in the central exclusive production of tensor $\chi_c(2^+)$ meson

TL;DR

This paper investigates the nonperturbative and spin effects in the central exclusive production of tensor _c(2^+) mesons in high-energy proton collisions, comparing theoretical predictions with experimental data and analyzing different gluon distribution models.

Contribution

It provides a detailed derivation of the production amplitude within the $k_t$-factorisation approach and compares various unintegrated gluon distributions, including nonperturbative effects and polarization contributions.

Findings

Good agreement with Tevatron data using specific UGDFs.

Nonperturbative gluon transverse momenta dominate the amplitude.

Higher spin states contribute significantly to the _c(1^+) and _c(2^+) channels.

Abstract

We discuss central exclusive production (CEP) of the tensor $\chi_c(2^{+})$ meson in proton-(anti)proton collisions at Tevatron, RHIC and LHC energies. The amplitude for the process is derived within the $k_t$-factorisation approach. Differential and total cross sections are calculated for several unintegrated gluon distributions (UGDFs). We compare exclusive production of all charmonium states $\chi_c(0^+),\,\chi_c(1^+)$ and $\chi_c(2^+)$. Equally good description of the recent Tevatron data is achieved both with Martin-Ryskin phenomenological UGDF and UGDF based on unified BFKL-DGLAP approach. Unlike for Higgs production, the main contribution to the diffractive amplitude of heavy quarkonia comes from nonperturbative region of gluon transverse momenta $Q_{\perp}<1\,\GeV$. At $y \approx$ 0, depending on UGDF we predict the contribution of $\chi_c(1^+,2^+)$ to the $J/\Psi + \gamma$ channel to be comparable or larger than that of the $\chi_c(0^+)$ one. This is partially due to a significant contribution from lower polarization states $\lambda=0$ for $\chi_c(1^+)$ and $\lambda=0,\,\pm 1$ for $\chi_c(2^+)$ meson. Corresponding theoretical uncertainties are discussed.